Not written, rather conveyed through centuries of oral tradition, for Homer anyway.

Writing reappeared fifty years after the time of Homer, within a hundred years or so bringing on the “golden age” of Greek philosophy, if that tells you anything.

Homer did not invent the Iliad, though it is strongly arguable he and his “school” did wholly invent the Odyssey, at least the story in which lore was woven.

The Iliad was a small part of the “epic cycle”, which only survived in fragments referenced by later literary discussions.

All this said, there is some truth to what you have highlighted. A grand epic is one which speaks to an age, and it takes generations of telling to process this into a mind worthy of its ultimate composure.